1. Field of the Invention
This invention relates to methods of modulating large-power light beams using fiber Raman amplification, especially for the fast modulation thereof. Accordingly, it is a general object of this invention to provide new and improved methods of such character.
2. General Background
The Raman effect is defined by the American Heritage Dictionary of the English Language, 1969, as: physics. The alteration in frequency and random alteration in phase of light scattered in a material medium. [Discovered by Sir Chandrasekhara Zenkata Raman (born 1888), Indian physicist.]
Long-distance light transmission in optical fibers is often limited by loss therein, a result in part of the limited power available from the light sources that are typically used, i.e., diode lasers. Light amplification by stimulated Raman scattering in fibers has been proposed as a technique for amplifying light back up to usable levels and experimental studies demonstrating gains exceeding 20 dB have been reported by Y. Aoki et al., "Efficient backward and forward pumping cw amplification for InGaAs laser light in silica fibers", Electronics Lett., 19, 1983, p. 620. However, Raman amplification has been limited by the lack of sufficiently powerful pump sources in that several hundreds of mW to one watt is required to be coupled into a fiber at approximately one Raman Stokes Shift, i.e., 450 cm.sup.-1 in inverse wavelength units, above the signal wavelength. For example, a non-existent pump source with several hundred mW to one watt at 1.24.mu.m is required for a signal wavelength of 1.3.mu.m.
An alternative method of long distance light transmission in optical fibers uses a large-power (hundreds of mW to 1W) laser such as a 1.32 .mu.m YAG laser as a communications signal source to overcome the fiber loss problem. An external modulator for the laser, however, is required that is capable of handling the involved optical powers and modulating same at high enough speed e.g., hundreds of MHz to GHz. Of the prior art modulators, the optical powers involved dictated the use of bulk, rather than guided-wave, modulators. For broadband applications, electro-optic types are desirable but require substantial drive powers. This can be seen from the figure of merit (corresponding to full amplitude modulation, i.e., an electro-optic retardation of .pi. radians),representing the drive power per unit bandwidth, at best about 10 mW/MHz as reported by I. P. Kaminow and T. Li, "Modulation Techniques", Optical Fiber Telecommunications, S. Miller and S. Chynoweth, eds. (New York, Academic Press, 1979), chapt. 17, p. 569. For modulation at 1 GHz, then, a very impractical 10W drive is required.